Chemically strengthened glass parts have become important for a variety of applications, including resilient, shatter-resistant and scratch-resistant, touch-enabled, protective flat cover windows for smart phones and tablets. These glass parts are thinner and lighter than thermally tempered glass yet tougher due to the high surface compression (for example, on the order of 8×108 Pa) achievable through an ion-exchange process.
The quick adoption, continuous improvement and dramatic market growth of such flat-glass products were boosted by the availability of quick nondestructive techniques for measuring the two major parameters of the stress profile: the surface compressive stress (CS) and depth of layer (DOL). Such measurements can be made using a commercially available high-resolution prism-coupling system, such as the FSM-6000LE made by Orihara Industrial Co., Ltd and sold by Luceo, both of Japan. The third critical parameter, the center tension (CT), may be inferred by invoking a force balance requirement between the compressive and tensile forces.
A prism-coupling system captures angular coupling spectra (“mode spectra”) of transverse-electric (TE) and transverse-magnetic (TM) optical propagation modes of the ion-exchanged region. The stress is extracted from the difference between the two spectra by using the stress-optic coefficient (SOC). Due to the small SOC (˜3×106 RIU/MPa, wherein RIU stands for refractive index units), the stress-induced part of the refractive index represents a small difference between two much larger index numbers. Consequently, the magnitude and shape of the stress profile are strongly affected by small errors in the recovered TE and TM profiles. To minimize such errors, high-resolution capture of the TE and TM mode spectra is necessary.
The excellent strength properties of chemically strengthened glass parts make them desirable replacements for existing curved glass parts, such as test tubes, and for non-flat external glass or plastic parts of personal electronic devices. However, quick nondestructive measurement of the TE and TM mode spectra on such curved parts for the purpose of measuring one or more characteristics, such as the stress profile or some of its critical parameters, has proven problematic.